Field emission electron gun

ABSTRACT

A field-emission electron gun wherein, during the state under which an electric field is formed between the cathode and the first anode to thus emit electrons, a heating current is supplied to a filament mounted on the cathode, to steadily heat the cathode so as to hold it at a certain fixed temperature within a range of from 100*C. to 500*C.

United States atent 11 1 Nomura 11 June 3, 1975 [54] FIELD EMISSION ELECTRON GUN 3,784,815 1/1974 Coates ct a1. 250/311 7 [75] Inventor: e o ura, K sut Japan 3,786,268 1/1974 Nomura -50/311 73 Assi nee: Hitachi Ltd., Ja n g pa Primary Examiner-Maynard R. W1lbur [22] Filed: Ju 1, 9 Assistant Examiner-J. M. Potenza [21] Appl No; 368,520 Attorney, Agent, or FirmCraig & Antonelli [30] Foreign Application Priority Data ABSTRACT June 6, 1972 Japan 47-56910 A field-emission electron gun wherein, during the [52] US. Cl 315/94; 250/311 state under which an electric field is formed between [5 Cl. th th d d th fi t anode t th l t [58] Fleld of Search 315/31 94; 250/31 a heating current is supplied to a filament mounted on 250/310, 307 the cathode, to steadily heat the cathode so as to hold it at a certain fixed temperature within a range of from [56] References Cited 100C to 500C UNITED STATES PATENTS 3,678,333 7/1972 Coates 315/31 5 Claims, 7 Drawing Figures 2 w 4b W1 T'MUJTEH JUH 3 I975 SHEET TIME (min) 0 & EH58 52mm 20586 BEE 6 2 e@ m M Q mw flW FS. 2b 4 3 .5 2 Pa H iTTET-mmm m5 8.887.885

SHEET FIG 6 :60 260 300 400 500 CATHODE TEMPERATURE (C) FIG 7 I08 200 360 460 560 860 760 800 9'00 CATHODE TEMPERATURE (C) FIELD EMISSION ELECTRON GUN BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a field-emission electron gun. More particularly, it relates to a fieldemission electron gun in which a cathode is steadily heated to a fixed temperature, whereby the electron current to be emitted from the cathode is stabilized.

2. Description of the Prior Art The field-emission electron gun is so constructed that a great field for drawing out electrons is applied to the surface of a cathode which is placed in a vacuum, to thus cause electrons to emit from the cathode surface. The electron current density of the electron gun of this type is much higher when compared with that of a prior art electron gun of the thermionic emission type. In addition, the energy width possessed by the emitted electrons is smaller than in the thermionic-emission electron gun. For these reasons, the field-emission electron gun is recently regarded as very important as an electron gun for electro-optic devices. It has, however, the disadvantage that since the emission electron current density is sensitively affected by surface conditions of the cathode, the period of time during which a stable electron current is acquired is extremely short.

FIG. 1 illustrates the changes-versus-time of an emitted electron current i during the state under which a cathode is held at room temperature without being heated in a prior art field-emission electron gun. When the electron emission is effected from a cathode which has been cleaned at the beginning of operation, the work function of the cathode surface increases on account of the adsorption of residual gas, and the emission electron current decreases abruptly. After a while, the decrease in the emission electron current slows down. When the gas-adsorbed condition approximate to a monoatomic layer is reached, a stable electron current is produced for a short time (T) as seen in the figure.

Upon completion of the stable state, the electron current becomes unstable, and its magnitude increases gradually. These phenomena are caused by the adsorption of excessive gas and the generation of microscopic disorders on the cathode surface as results from ion bombardment. It is thought that the electron current fluctuates due to the adsorption of an excessive amount of gas, and that the electron current density increases because high electric field strengths act locally on protrusion-shaped parts having arisen as the result of the bombardment on the cathode surface by positive ions created by the emitted electron current. If allowed to stand, the increase in the electron current density gradually becomes larger and more erratic with time, finally leading to destruction of the cathode due to the Joule heat and the strong electric field.

It is therefore required that upon lapse of the period T of the stable state, the electric field for effecting the electron emission is cut off to stop the operation of the electron gun, and that the cathode is heated to a high temperature to perform restoration or flashing (cleaning and smoothing) of the surface thereof.

With the prior art field-ernission electron gun, the stable characteristic is attained only for the period T as stated above. How the stable state may be maintained for a long period is accordingly a very important subject. It has therefore been hitherto endeavored to enhance the degree of vacuum of the device as far as possible, so as to maintain the conditions of the cathode surface as uniform as possible. Besides, in order to avoid contamination due to the adsorbed gas, there has been proposed a method by which the cathode is heated above 1,000C. during the operation of the electron gun. According to the method, the cathode surface is kept clean, and the minute disorders in the surface do not appear. The method, however, had disadvantages, as mentioned below.

1. Since a strong electron emitting field acts on the cathode surface, a deformation of the cathode termed build-up arises.

2. Atoms are continually moving on the cathode surface, so that the emission electron current flickers.

3. The energy width of emitted electrons spreads, which deprives the field-emission electron gun of the characterizing feature that the energy width is narrow.

Because of these disadvantages, the proposed method is not readily adopted in practice. The conventional field-emission electron gun is used with the cathode left at room temperature on the condition that the degree of vacuum of the device is brought to the best possible value. As previously stated, the stability of the electron current is not satisfactory. Since, in addition, a high-temperature heating operation for cleaning the cathode is frequently required, the tip end of the cathode becomes blunt early in the operation, and the life of the electron gun is therefore quite short.

SUMMARY OF THE INVENTION It is accordingly an object of the present invention to provide a field-emission electron gun which can emit a stable electron current for a long time without heating the cathode to a high temperature, as mentioned above.

A further object of the present invention is to provide a field-emission electron gun which has a long life without the necessity for effecting a cleaning operation for the cathode.

In order to accomplish these objects, the fieldemission electron gun of the present invention is characterized in that the cathode is continually heated to a low temperature of C.- 500C.

The other objects and features of the present invention will be apparent from the following detailed description when read in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram of a curve showing the changeswith-time of an emission electron current from a cathode held at room temperature;

FIG. 2 is a schematic longitudinal sectional view showing an example of the construction of a fieldemission electron gun according to the present invention;

FIG. 3 is a diagram of a curve showing the changeswith-time of the emission current in the case where, after having the cathode surface cleaned, the electron gun is operated with the cathode held at 250C;

FIG. 4 is a diagram of a curve showing the changeswith-time of the emission current in the case where the cathode is allowed to stand at room temperature after the operation, and then the electron gun is operated without cleaning the cathode surface;

FIG. is a diagram of a curve showing the changeswith-time of the emission current in the case where the cathode is kept at 250C. even after cessation of the operation, and thereafter the operation is restarted;

FIG. 6 is a diagram of a curve showing the rate A of gradual increase in the electron current per unit time as taken versus the cathode temperature; and

FIG. 7 is a diagram of a curve showing the rate B of high-frequency noises as taken versus the cathode temperature.

PREFERRED EMBODIMENT OF THE INVENTION In FIG. 2, showing an embodiment of the present invention, reference numeral 1 designates a cathode, which is steadily heated by supplying a heating current i to a filament 2 from a power source 3. A high-voltage power source 6 is connected between the cathode 1 and the first anode 4, to form an electric field for drawing out electrons from the cathode 1. Connected between the first anode 4 and the second anode 5 is an accelerating power source 7, which serves to accelerate the electrons.

The electron beam current i,,, which is field-emitted from the cathode 1, passes through an opening portion 4a of the first anode 4. Further, it is accelerated by an electric field formed between the first anode 4 and the second anode 5. Thereafter, it is ejected from an opening portion 5a of the second anode 5.

Studies on the effect of heat radiation of the cathode l were made in such'a way that, in the electron gun of the construction as stated above, the surface 4b of the first anode 4 on the side of the cathode 1 was coated with a fluorescent material so as to permit observation of the image of the emitted electron current i, from the cathode 1. Then, the following results were obtained:

1. Similarly to the case where the cathode is left at room temperature, a stable electron current is established during the state under which the cathode 1 is saturated with an appropriate amount of adsorbed gas.

2. The effect of avoiding the adsorption of excessive gas and holding the cathode surface smooth by the movements of atoms begins to appear at a cathode temperature of approximately 100C. It increases with the temperature.

More specifically, as illustrated in FIG. 6, when the cathode temperature is made higher, the gradual increase of the emission electron current decreases more. In the figure, character A denotes the rate of the increase of the electron current per unit time as taken versus the cathode temperature, the increase occurring after the time when the cathode surface has been saturated with the appropriate amount of adsorbed gas. It is understood from the graph that the cathode need be heated to at least 100C. in order that the emission electron current may not increase by 30% or more even with a lapse of time of 1 hour.

3. When the cathode temperature is further raised in the foregoing manner, the surface motions of tungsten atoms become violent, and hence, the gradual increase of the emission electron current remains substantially constant as shown in FIG. 6. Noises at high frequencies, however, begin to arise. The rate B of the highfrequency noises increases as in FIG. 7 versus the cathode temperature. It is understood from the figure that, in order to suppress the rate of the noises to 5% or less, the cathode should not be heated above 500C.

To sum up the foregoing, the cathode temperature of or above C. is required in order to make the gradual increase of the electron current 30% or less per hour, while the cathode must not be heated above 500C. in order to restrain the rate of the highfrequency noises of the electron current to or below The allowable varying range for stably maintaining the emission electron current as actually required in a device employing a field-emission electron gun, such as a scanning type electron microscope, is at most 30% per hour as to the gradual increase of the electron current and is at most 5% as to the rate of the highfrequency noises at the electron current.

As a result, it is apparent that a very stable electron current is achieved by steadily heating the cathode so as to retain the cathode temperature at a fixed one lying between 100C. and 500C.

Reference is now made to FIG. 3 which illustrates the change-versus-time of the emission electron current in the case where the field emission is carried out with the cathode temperature held at 250C.

The change-versus-time in the case where the field emission is effected during the state under which the cathode is held at the room temperature, have been already explained with reference to FIG. 1. As understood by comparing FIGS. 1 and 3, when the electron gun is operated with the cathode l maintained at 250C, the period of the stable operation in which the emission electron current is substantially constant becomes several tens or more times as long as that in the operation at room temperature.

Since the cathode of the prior art field-emission electron gun is kept at room temperature, large quantities of gas are adsorbed. When the operation is started without conducting the cleaning of the cathode surface, the excessive gas moves about on the cathode surface, with the result that the emission electron current is very unstable, as illustrated in FIG. 4. It is therefore necessary that, at the beginning of the operation, the cathode is first heated to a high temperature to thereby clean its surface. It is true, as previously stated, that this procedure renders the cathode blunt, to shorten the life of the electron gun.

According to the electron gun of the present invention, even during cessation of the operation, the cathode 1 can be held at the same temperature as that during the operation. Therefore, the adsorption of the excessive gas is avoided for the period, and the cathode surface is retained in the stable state. In the case where the cathode 1 is kept heated to 250C. during the cessation of the operation and where the operation is started again at the cathode temperature, the subsequent changes-versus-time of the emission electron current become as shown in FIG. 5. In this case, a stable emission current is produced under quite the same conditions as in the preceding operation.

As described above, in accordance with the present invention, a stable emission electron current is obtained. Besides, the cleaning procedure for the cathode becomes unnecessary, so that the life of the electron gun is much increased. It has been made certain that the deformation of the cathode is not created within the heating temperature range of the present invention, and the energy width of the emission electron current scarcely varies from that in the case of the roomtemperature operation.

What is claimed is:

l. A field-emission electron gun comprising a cathode, a first anode, a second anode, a source of a first electric potential connected between said cathode and said first anode to form an electric field for drawing out electrons from said cathode, a source of a second electric potential connected between said first anode and said second anode to form an accelerating electric field for accelerating the electrons drawn out from said cathode, and power supply means for continuously heating said cathode to a fixed temperature within a range of from 100C. to 500C. during a state under which said electrons are being emitted from said cathode.

2. A field-emission electron gun as defined in claim 1, further including a filament for heating said cathode, said power supply means including a power source supplying a direct current continuously to said filament.

3. A field-emission electron gun as defined in claim 2 wherein said filament power source supplies a filament current having a value to steadily heat said cathode to a fixed temperature of 250C.

4. In a method of stabilizing an emission electron current in a field-emission electron gun, during a state under which electrons are being emitted by an electric field formed between a cathode and a first anode, the improvement comprising the step of supplying a heating current continuously to a filament attached to said cathode to steadily heat said cathode so as to hold it at a fixed temperature within a range of from C. to 500C.

5. The method defined by claim 4 wherein said cathode is supplied with a heating current continuously to heat said cathode to a temperature of 250C. 

1. A field-emission electron gun comprising a cathode, a first anode, a second anode, a source of a first electric potential connected between said cathode and said first anode to form an electric field for drawing out electrons from said cathode, a source of a second electric potential connected between said first anode and said second anode to form an accelerating electric field for accelerating the electrons drawn out from said cathode, and power supply means for continuously heating said cathode to a fixed temperature within a range of from 100*C. to 500*C. during a state under which said electrons are being emitted from said cathode.
 1. A field-emission electron gun comprising a cathode, a first anode, a second anode, a source of a first electric potential connected between said cathode and said first anode to form an electric field for drawing out electrons from said cathode, a source of a second electric potential connected between said first anode and said second anode to form an accelerating electric field for accelerating the electrons drawn out from said cathode, and power supply means for continuously heating said cathode to a fixed temperature within a range of from 100*C. to 500*C. during a state under which said electrons are being emitted from said cathode.
 2. A field-emission electron gun as defined in claim 1, further including a filament for heating said cathode, said power supply means including a power source supplying a direct current continuously to said filament.
 3. A field-emission electron gun as defined in claim 2 wherein said filament power source supplies a filament current having a value to steadily heat said cathode to a fixed temperature of 250*C.
 4. In a method of stabilizing an emission electron current in a field-emission electron gun, during a state under which electrons are being emitted by an electric field formed between a cathode and a first anode, the improvement comprising the step of supplying a heating current continuously to a filament attached to said cathode to steadily heat said cathode so as to hold it at a fixed temperature within A range of from 100*C. to 500*C. 